This invention relates to a solar tracking device, and more particularly to a solar tracking device which operates under the control of a constant speed drive mechanism to cause a solar pointing axis to track the movement of the sun as it moves across the earth's sky. The invention further relates to a solar energy collection system which may be optimally positioned for collecting maximum solar energy at any time during the day.
Solar tracking devices in the prior art have taken a number of approaches for directionally tracking the sun. One method, used in most astronomical telescopes, is referred to as a polar mount, and has two axes of freedom. The first axis, commonly called the polar axis, is typically carried by coaxially aligned bearings on two piers or towers of dissimilar height that lie in the plane of the earth's spin axis. The height of the towers is adjusted to cause this polar axis to point at the earth's celestial pole. The second axis of freedom on this type of device is attached orthogonally to the polar axis. Typically, a payload such as a telescope or solar collector, is attached to the orthogonal axis. Once the device is set to the appropriate declination of the celestial object to be tracked, it is driven by only the polar axis in order to remain aligned. This mounting method and approach is eminently successful for observatory instruments, but impractical for solar energy collection. When a large, flat rectangular solar array is placed on a polar mount, the two support towers must be unnecessarily tall. This is essential in order to avoid ground interference when the array is pointed east and west. The increased height creates architectural and structural problems and significantly adds to the cost of material and installation. The singular advantage of a polar mount for solar energy collection is that only the polar axis need be driven at a constant rate of one revolution per solar day in order to accurately track the sun.
A second method for directing a solar array at the sun is commonly achieved with what is called altazimuth mounting. This mounting also has two degrees of freedom. The solar array, or solar energy collecting device, is attached to the first (altitude) axis. This axis normally lies in a horizontal plane and it permits the solar array to be moved vertically to the proper elevation angle of the sun at any particular time. The second axis of the altazimuth mounting carries both the elevation axis and solar array about a locally vertical axis. For a given moment in time, the azimuth axis must be moved to the proper azimuth angle of the sun. This type of mounting is typically found in a surveyor's transit or naval gun mount, and the mounting has several advantages over the polar mount for solar tracking purposes. Unlike the polar mount, when the solar array is pointing east toward the morning sun, or west toward the evening sun, the bottom edge of a typical rectangular energy collection array is always parallel to the horizon. Consequently, the device can be considerably lower in height than a corresponding polar mount device. This leads to a device in which wind load stresses are reduced. Further, the structural members are usually in compression rather than cantilevered stress as is found in a polar mount, which results in a distinct advantage in construction costs for material required in a given size application. Also, its structural shape permits it to be more easily placed on a flat building roof than a corresponding polar mounting apparatus.
However, the primary disadvantage of the altazimuth mounting severely compromises the foregoing advantages, because it provides a significant problem in driving the axes of the mount to follow the sun. The mount must be continually driven on both the elevation and azimuth axes at a nonuniform rate in order to accurately track the sun. Early in the morning the tracking motion is largely elevational with very little azimuth travel. Elevation motion decreases to zero when the sun crosses the prime meridian at high noon, at which time the motion is entirely azimuthal. The reverse relative motions are repeated in the afternoon towards sunset, and the problem can be solved by only two methods. Both of these methods require a motor drive for each axis, which motors must be continually directed by either cybernetic, or computer stored azimuth and elevation positions, or some form of optical pointing device such as an electro optic sighting telescope which may be used to develop a motor error drive signal. The foregoing engineering problem includes the task of delivering the amplified elevation signal to the elevation motor, which frequently requires slip rings. This can be readily accomplished for a sophisticated and expensive device such as a radar missile tracker, but presents a prohibitive cost when it is desired to utilize the tracking device in a simple, inexpensive solar tracking system.
It is an object of the present invention to provide a low cost solar tracker in which, by purely mechanical coupling, a constant speed drive input can be resolved into two non-constant components of motion that will cause an altazimuth mount to precisely follow the sun. It is a further object to achieve the foregoing motion from a single, synchronous drive motor, and to avoid the requirement of both an elevation and an azimuth drive motor and the consequent electronic or electro optical feedback system for developing motor error drive signals. It is yet another object of the invention to provide such a low cost solar tracking system which is of relatively simple construction so as to permit maintenance and repair to be accomplished with relative ease.